I don't remember a lot about the Webtoon since it's been a while. The female lead has black hair and so does her older brother. I believe she went back in time to when she was younger. Her parents both die and her brother took over their Title (I believe he is a Duke but not sure).
I'm pretty sure they didn't get along at first but she tries to get close to him by bringing him tea and stuff while he's working and they end up becoming very close. FL is sick where she had to have mana(?) transferred to her. I think it's a disease where she runs out of mana. Her brother has been transferring mana to her. She goes somewhere in a carriage and meets the Male Lead (ML) who is a prince and I believe everyone is afraid of him and calls him a devil or something because he has really strong mana. He has light colored hair. FL asks him to destroy a tunnel that connects to her home (I think in the future the king or something uses it to attack them that's why she wants to destroy it but not sure) also she smokes a pipe.
The ML because a little obsessed with her and her brother doesn't like him getting close to her. Also when she went back in time she realized her maid was intentionally ruining her relationship with her brother so she gets rid of her. That's all I can really remember.